a. Field of Invention
The invention relates to Superconducting Quantum Interference Device (SQUID) and, more particularly, to a method and apparatus for quantifying the output of a SQUID sensor by subranging.
b. Background of the Invention
The Josephson Effect, a quantum phenomenon, was largely formulated by the Nobel Prize Laureates Brian D. Josephson and Philip W. Anderson. The Josephson Effect entails a flow of electric current as electron pairs, called Cooper pairs, between two superconducting materials that are separated by an extremely thin insulator. This arrangement is called a Josephson junction. This phenomenon in superconductors is the basis of the Superconducting Quantum Interference Device (SQUID). SQUIDS are highly sensitive magnetic flux detectors that can be used for measuring temperature, displacement, magnetic fields, etc. Indeed, they are the most sensitive detectors of magnetic flux currently known. The DC superconducting quantum interference device (SQUID) consists of two superconductors separated by thin insulating layers to form two parallel Josephson junctions (another implementation, the RF SQUID, requires only a single junction). The device may be configured as a magnetometer to detect incredibly small magnetic fields—small enough to measure the magnetic fields in living organisms. Alternatively, SQUID devices can be adapted to measure temperature, displacement, etc. Despite the existing sensitivity of these devices, there is room for improvement.
For example, the potential for even more sensitive measurements of displacement has profound implications for several high-impact gravitational physics and particle physics projects in space. Example applications include the detection of gravitational waves, the test of the equivalence principle, the search for the postulated “axion” particle, and the test of the inverse square law of gravity, all of which rely on the detection of small displacements. If the displacement sensor is sensitive enough to observe the quantum mechanical zero-point motion of a test mass, it will be possible to make a direct examination of the Heisenberg uncertainty principle.
SQUID temperature sensors generally comprise a coil wound on a core made of a material which has a temperature-dependent magnetic susceptibility in the temperature range of interest. The core and the coil are placed in a DC magnetic field provided either by a permanent magnet or by a flux trapped inside a superconducting outer wall. The coil is connected to an input coil of a SQUID. Changing temperature leads to changes in susceptibility of the core and to changes in the magnetic flux detected by the SQUID, which is capable of detecting small magnetic flux changes.
FIG. 1 illustrates a generic SQUID system employing a flux pickup coil 12 coupled to an inductive input coil 14. The flux pickup coil 12 detects directly-applied flux and couples currents into the input coil 14. In practice, the flux pickup coil 12 is typically wrapped around a paramagnetic salt pill. The salt pill is thermally linked to a sample of interest for indirect thermal measurement of the sample via measurement of the magnetic susceptibility of the paramagnetic salt. The coupled currents generate flux that couples into the SQUID loop 20 for detection. See, for example, “Absolute Temperature Measurement”, IBM Technical Disclosure Bulletin, April 1989, vol. 31, No. 11, pp. 396-397 TBD-ACC-No.:NN8904396.
Using SQUIDs, researchers have made the most sensitive measurements of magnetic field, temperature, displacement and pressure. There are a variety of digital readout schemes for measuring the output from SQUID-based single sensors and sensor-arrays. The response of a SQUID device to the enclosed magnetic flux is a periodic function of the flux. A common implementation of a SQUID sensor readout uses feedback to inject additional magnetic flux into the sense loop of the SQUID device to maintain the total flux in the loop constant. The SQUID controller output is then proportional to this injected additional flux. When the input signal causes the limit of the feedback current to be exceeded, the controller is “reset”. The magnitudes of the magnetic fluxes before and after the reset differ by an integer multiple of a fundamental unit. Φn=h/2e. Thus, flux can be measured across a wide dynamic range by keeping track of these flux resets via a hardware or software counter.
A more detailed diagram of a digital SQUID is shown in FIG. 2. The magnetic flux in SQUID 12 causes a voltage (V) to appear across it. Measurement of the characteristics of a signal current can be made by measuring the magnetic flux it produces in a pick-up coil 10. The analog SQUID 12 senses pick-up coil 10, and provides an output that is coupled to a clocked and gated comparator 18. The comparator 18 digital output provides both the desired flux measurement as well as linearizing feedback that is coupled back to the pick-up coil 10.
Rather than a hardware counter (clocked and gated comparator) as above, some applications employ a software counter. While adequate for some applications, these existing SQUID digital hardware/software counters suffer from a few drawbacks. If the temperature is changed rapidly, or the temperature noise is high, the counter hardware/software tends to lose flux count. With a software counter, the temperature reading is lost entirely if the software is reset/restarted. A multiplexed SQUID controller aggravates the problem, as the flux reading is performed less frequently.
A few different approaches have been attempted to correct this. One such attempt attaches binary carry lookahead (BCL) circuits attached to the counted for higher-speed operation to adapt to rapid temperature fluctuations. See, Double Relaxation Oscillation SQUID with a 4JL On-Chip Digital Flux Locked-Loop Circuit, H. Myoren, M. Goto, T. Taino, S. Takada, Saitama Univ.; K. Kikuchi, H. Nakagawa, K. Tokoro, M. Aoyagi, AIST
Another promising technique is digital subranging. Subranging has been applied in other contexts. For example, U.S. Pat. No. 6,556,153 to Cardamone issued Apr. 29, 2003 shows a system and method for improving encoder resolution using software. Dual measurements are taken: 1) a table is populated with position information based on a sample of the signals from a position sensing device, such as to provide the position information for the table; and 2) an indication of position is determined as a function of velocity and time.
U.S. Pat. No. 6,377,110 to Cooper (Keystone Thermometrics) issued Apr. 23, 2002 shows a low-cost temperature sensor circuit for silicon thermometry that possesses a high degree of linearity and a wide dynamic range. The circuit uses a mixed-signal approach in which zero offset and desired output voltage are set, independently of each other. Slope (span) is set at a full scale temperature different from the first temperature.
Subranging has only recently been attempted in SQUID sensor arrays to produce different “course” and “fine” outputs. This improves the dynamic range as well as the speed or “slew rate” of the output circuitry (dynamic range is the ratio of the maximum signal that can be measured to that of the minimum signal that can be measured, the slew rate is the time rate of change of signals).
U.S. patent application Ser. No. 20020060635 by Gupta, published May 23, 2002, which discloses a subranging architecture using digital SQUID technology to design systems with larger dynamic range, higher resolution and larger bandwidth than existing systems. As shown in FIG. 3, the architecture uses a first SQUID device SQ1 coupled to a first inductive section, and a second SQUID device SQ2 coupled to a second inductive section. The first SQUID is designed to produce “coarse” (large amplitude, low resolution) output signals and the second SQUID is designed to produce “fine” (low amplitude, high resolution) output signals in response to the analog input signals. The output signals of the first SQUID SQ1 are coupled to a first comparator 101 having an output for producing a first quantized output signal which is coupled back to the input coil L1. The output signals of the second SQUID SQ2 are coupled to a second comparator 12 having an output for producing a second quantized output signal which is also coupled back to the input coil. The first and second inductive sections L1, L2 are connected in series or in parallel, the inductance of the second inductive section L2 being greater than the inductance of the first inductive section L1 for causing the first SQUID SQ1 and the first comparator L1 to produce a “coarse” ouput and the second SQUID SQ2 and the second comparator L2 to produce a “fine” output. The coarse resolution SQUID SQ1 is first used to cancel the flux in the pick-up loop until the amplitude of the input signal is below a “coarse” resolution step. Then, the fine resolution SQUID SQ2 takes over when the flux in the pick-up loop is smaller than the resolution of the coarse SQUID SQ1. The subranging scheme proposed by the Gupta '635 application essentially uses digitized feedback to “null” the flux through the pick-up loop l.p. This “nulling” is accomplished by sequentially introducing fluxons by superconducting “gates”. Unfortunately, as described more fully (below) this still limits both slew rates and dynamic range.
It would be far more advantageous to provide a subranging scheme that leaves linearizing feedback with the individual SQUID sensors rather than introducing feedback current into the input coil. This would allow the readout of individual SQUID sensors to be done simultaneously, not sequentially, thereby increasing slew rates and dynamic range.